Stereophonic moving magnet phonograph pickup



Feb. 12, 1963 E. AHRl-:Ns ETAL 3,077,521

s'rEREoPHoNIc MOVING MAGNET PHoNoGRAPx-x PICKUP v Filed Nov. e, 1958 FIG.|

Erhard i Ahrens, Kiel, Schleswig-Holstein,

'-'graphli' pickup for stereophonic 3,077,521 STEREOPHQNIC ll/iOiIll'G MAGNET PHNOGRAPH PICKUP Hans-Robert Khn, Kiel-Gaarden, Schleswig-Holstein, Richter,V (icl, Schleswig-Holstein, Germany, assignors, by mesne assignments, to Shure Brothers, Incorporated, a corporation of Illinois Filed Nov. 6, 1958Ser. No. 172,233 Claims priority, application Germany Mar.

1S Claims. (Cl. 179-1-100Al) `ur yinvention-relates toan'electromagnetic phonogrooved records whose two 'soundtracks are located on the respective opposite side iwalls' of'a single groove. More particularly, the invention concerns a stereophonic pickup in which the respective recordings of the two ltracks are reproduced with the aid Aof two'induction coils which produce respective output i'voltages in response to the vibratory motion of a single 'stylus in two planes intersecting each other at an angle of -approximately 90 and inclined about 45 to the surface plane of the record disc. A

It is known to use electromagnetic groove cutting defvices for `producing the above-mentioned twin-track grooves on a record disc. One of these known devices (British`Pa-tent 394,325, FIG. 9) comprises a magnetic Icircuit in form of a triangular yoke placed upon one of its corners, the lowermost corner portion being open to fo'rm an air gap. Each ofthe two upwardly extending limbs'of this magnetic circuit carries a main winding andan'auxiliarywinding. A movable armature of magnetically conducting material which carries the cutting stylus is located in the gap between the pole ends of the two limbs and is dispiaceable in the longitudinalY direction-of both limbs. The main coil on one of the respec- -tive limbs and the auxiliary coil on the other `limb are electrically connected in series to form part of the two v:":or'nponent cutting systems. Since the coils of both corn- 'pon'ent systems are thus inductively coupled with one and the same magnetic circuit, the channel separation ordevvcoupling of the two cutter control systems is farfrom satisfactory.

It is -an object of our invention to devise a magnetic pickup for stereophonic sound records that affords a xgreatly improved separation between the respective reproductions of the two sound tracks than could be obtained with a reproducing pickup based upon the abovementioned design principles of the known record cutting V4device. Another object of our invention, vconjointly with 'the foregoing, is to reduce the oscillating mass ofthe vrstylus assembly and to greatly minimize its detrimental effects upon the quality of reproduction. `Still another object in conjunction with those aforementioned is Ato .clevise a stereophonic pickup of the magnetic type in which 'the oscillating stylus assembly asa whole can be removed or linserted with particular ease for replacement purposes.

According to one of the features of our invention, Athe pickup comprises two mutually independent magnetic 1circuits each having a yoke `with two poleportions spaced from each other in a direction lthat extends at an angle of approximately 90 to the spacing direction of the other 'two pole portions so as to jointly forma four-pole gap space. The spacing direction of a pair of pole pieces or pole portions is the direction of a line perpendicular to a plane between the pole portions, which plane is so located that if a magnet was moved in or parallel to the plane there would be substantially no magnetic change in the yoke or no response in the electric channel circuit associated therewith. The induction coils of the two electric channel circuits are mounted on the respective yokes and electrically separate from .each other thus providing for a United States Patent andr Erhard 5 able stylus assembly applicable for `"pickup, be exchanged for replacementl purposes.

satisfactory separa-tion between the two channels. The

lstylus assembly of the pickup is providedwith an elongated stylus arm and has an elongated and longitudinally magnetized permanent magnet tirmly joined with the arm in coaxial relation thereto. The stylus tip of the assembly 'is rigidly mounted on the end of the arm remote from the magnet. Furthermore, the above-mentionedv stylus assembly as a unit is exchangeably seated in the gap space between the four pole'portions in coax1al relation to'that relative to which the assembly unit is slidingly respace,

axial direction. The stylus movable and insertable in the `armtogether with the'attachedv magnet andthe stylus 'tip are angularly displaceable about a pivot location within the gap space and preferably coincident or nearly coincident'with vthe gravity center of the stylus unit. As a "result, any detrimentaleifect of the entire oscillating'mass `upon the response to the sound tracks, can be keptextrem'ely slight and the entire oscillating portion of the including the stylus and the magnet, caneasily According to another feature of our invention, the exchangeable stylus assembly comprises a sleeve which jis nstuck into the gap space between the pole portions of the vmagnetic circuits and withinwhich the stylus arm with the attached magnet is pivotally displaceable asdescribed above. The sleeve is preferably givena portion whose cross section corresponds to that of the gap space so that this sleeve portion is frictionally held between the pole portions. The stylus arm and magnet areA preferably mounted within the just-mentioned sleeve portion by vmeans of la spacer of elastic material such as a ring of rubber which normally holds the magnet at equal spacing from the peripheral wall of the sleeve. According to a further feature of our invention the sleeve, partly seated in the gap space, has a top portion extending from the, seated portion along .the stylus arm and yabove the stylus tip, and the 4sleeve has an opening, "or'is entirely open beneath the top portion, so that the stylus tip protrudes downwardly for free movement in 'response to the undulation's ofthe soundtracks. As a result, the stylus arm and the appertaining partsof the oscillatingassembly are protected from abuse or .damage on the top as well as on both sides. However, the stylus can yieldingly'move into the sleeve if it is ever placed too forcefully onto the record disc. Hence, only the prot'e'ctive sleeve but not the styluscan be subjected to irnpact in the event the t'one arm carrying the pickup drops onto the record disc. Y

v'According to further features of the invention, thc proper and permanent mounting ofthe two magnetic circuit devices in correct position is improved by embedding the magnetic yokes and the appertaining coils within a body of casting resin. It is further preferable to provide arnetallic shield between the two magnetic circuits. The shield .consists preferably Vof magnetically Yconducting material such as available commercially underkthe trade name Mumetal, to prevent magnetic as wellas .electric interaction between the two magnetic circuits. T he metallic vshield can likewise be embedded vin the above-mentioned body of casting resin in 'order to be rigidly secured inproper position. a

The invention will be further described with reference to the embodiment of a stereophonic pickup according to the invention illustrated-by way of example on the accompanying drawings in which: 3 FIG. 1 is a verticallongitudinal section of the pickup with two induction coils.

FIG. 2 is a 'cross section of thesarne pickup parallel to the plane defined lby the Arespective longitudinal axes ofthetwocoils. M I y Y' y FIG. 3' is a longitudinal vertical section of an exchange# the pickup.

able material Aand has v dicular to each other loperative condition of the pickup,

- The two yokes 3 and ing -resin which forms vfound a slightly larger angle namely a FIG. 4 is a longitudinal vertical section of a modified form of such a stylus assembly.

FIG. 5 is a vertical cross'. section along the line A--B `in-FICllgand f i FIG. 6 shows schematicallyA one of thev two magnetic yokes of the pickup according to FIGS. land 2.

" .All illustrations are on enlarged scale.

#The illustrated stereo pickup is provided with t'wo inductively independent magnetic `circuits formed by respective yokes 3 and 4 of which each carries an induction coilfl or 2. Each yoke is formed of a strip of magnetizthe shape apparent for yoke3 in ofthe two yokes and hence the axes of coils 1 and 2 are perpen- (FIG. 2) and, in the illustrated intersect the horizontal plane of the record surface at an'angle of about 45. 4 and the appertaining induction coils 1, 2 are completely embedded in a Ibody 24 of casta protective enclosureY and also FIG. 6. The planes .respective longitudinal secures the While the planes of the two yokes in the illustrated ernbodiment intersect each other at an angle of 90, a somewhat dilferent angle may be provided. For example, we of about 100 preferable in some cases.

The yoke 3 has two elongated pole pieces 5 which extend in parallel opposite each other and are spaced from each other in the plane of thev loop-shaped yoke structure (FIG. 6). The yoke 4, having the same shape,` has two pole pieces 7 and 8 also extending parallel to each other and mutually spaced in the plane ofthe correlated loop structure (FIG. 2). The four pole pieces are arranged symmetrically about the point of intersection ofthe -two coil axes and jointly dene a gap space of generally square cross section and axially elongated shape, so that` the. gap space has the shape of a prism placed on edge (FIG. 2).

of the pole pieces are surrounded j by a rubber ring 9. Stuck into the gap space and vfricfallyof square shape. The sleeve 10 forms part of the exchangeable stylus assembly and will be described more in detail with reference to FIGS. 3 to 5. v

Mounted in the sleeve 10 o-f the stylus assembly is a stylus unit consisting of three firmly interconnected parts,

mits angular deflection of vthe stylus unit about a pivot point located between the pole Vpieces of the two yokes and substantially'coincident with the center of gravity of the unit. The arm 11 consists preferably of light metal such as aluminum. Y j

The portion'of sleeve llladjacent to the arm-16 and the stylus 14 forms a protective cover 13 which is down-I wardly open so that the stylus`14 can enter into engagement with the sound-track groove of therecord-carrier. The stylus 14 protrudes downwardly beneath the lower edgejof 'the protective cover 13 only to such an extent that it can yield behind the lower edge when the pickup or tone arm is dropped onto the [record or otherwise subjected'to excessivecontact pressure. 'Il-le' front end of the cover 13 is bent upwardly in order to facilitate gripping the sleeve by hand when sticking the sleeve into the gap space or pulling it out.

The rear end of sleeve 10 is se-aled -by means of a cup 17- land is additionally insulated and secured imposition by vmeans of a dabY 18 of paint or varnish. The hollow parts and rigid position relative to each other.

The longitudinal axis 31 (FIG. 1) of the gap -space is slightly inclined to the hori- 'zontal.` Theffree ends pieces, the above-described pickup space between rubber ring aiid cup with a fluid damping material such resonance. For increasing th damping eiect, the rear end of the magnet may be provided with damping vanes l) (FIGS. 1, 4, 5).

For securely preventing one magnetic circuit from magnetically and electrically inuencing the other, an angular shield member 22 metalsuchas available under the trade name Mumetal, is located betweenthe two yokes and the respective induction coils mounted thereon. The shield-22 is connected by electric leads (FIG. 2) with the two yokes and is also grounded when in operation.

As mentioned, the housing 24 (FIG. 2) of the pickup is filled with casting resin23 so that all parts of the magnet systems inthe housing, including the shield 22 are firmly held in position. When casting the resin into the housing, the sleeve 10' with the entire stylus unit is preferably removed from the gap space and i-s temporarily substituted by a plug of the same shape and size which is subsequently removed after hardening of the resin. The recess thus molded int-o the body of casting resin is thereafter available for snugly receiving the protective sleeve 10 with the stylus system.

By virtue of the particular orientation of the pole is designed for the of stereo sound recordings in which each lateral wall of a single groove is provided with one of the two ondulato-ry sound tracks. The walls of such a groove intersect the horizontal surface plane of the record carrier at an angle of approximately 45'and thus extend parallel to the-axes of the respective induction coilsfl and 2 or the respective planes of the two yokes 3 and 4. As a result, a deection of the stylus tip 14 in the direcupon the components of the respective magnetic circuits. As a result, the voltages generated in the induction coils correspond to the respective sound recordings of the two tracks. In this respect the pickup according to the invention secures an optimum amount of cross-talk attenuation, i.e. aA good set forth in the claims annexed hereto.

We claim:

1. A phonograph pickup cartridge comprising a cartridge body and a pair of spaced-apart pole pieces forming a gap there.- between and a coil arranged about said core, the orientaf tion of said pole pieces being such that they form a common gap with the gap spacing directions of the respectivepairs of pole pieces perpendicular to each other, said common gap forming an open-ended socket, and a subassembly frictionally and removably held in the socket,

of sh-eet metal,v preferably of and means including pieces for `stereophonic vibration of the armature "by the stylus.

2. A `pickup cartridge according to claim l wherein `,said pole pieces are embedded in a substantially rigid plastic block with a recess formed therein to snugly receive a portion of said subassembly, whereby said pole pieces and said subassembly are permanently maintained in proper orientation.

i3. A phonograph pickup cartridge comprising a cartridge body and a transducer system operatively mounted therein, `said transducer system comprising a pair of electromagnets each comprised of a core terminating in a pair of spaced-apart pole pieces forming va gap therebetween and a coil arranged about said core, the orientation of said pole pieces being such that they form a cornmon gap with the gap spacing directions of the respective pairs of pole pieces perpendicular to each other, and an armature-stylus subassembly, said subassembly comprising a generally sheet-form supporting member having a sleeve at one end mounted within said common gap, a bearing of an elastomeric material having an axially positioned aperture forming a socket tixedly mounted within said sleeve, an armature comprising a permanent magnet operatively mounted in said socket for angular vibration within said common gap substantially about a point intermediate the poles of said magnet, the magnetic axis of said armature being oriented substantially perpendicular to Vthe spacing direction of said pairs of pole pieces, and a stylus affixed to said armature for transinitting vibration thereto, whereby said subassembly is removable by grasping said supporting member and retracting said subassembly axially.

4. A phonograph pickup cartridge according to claim 3 wherein the cross-sectional configuration of said socket is substantially the same as that of said armature.

5. A phonograph pickup cartridge according to claim 3 wherein the electromagnets are shielded by a magnetic shield.

`6. In a pickup according to claim 3, said two cores forming respective loop structures extending in respective planes of angular relation to each other, and a grounding shield member of magnetically conducting metal mounted between said two cores kfor preventing magnetic and electric interaction of said two electromagnets, said shield member having angular shape and having two an-gle-leg portions substantially parallel to said respective two planes.

7, An armature-stylus subassembly for a magnetic pickup cartridge comprised of two eletromagnet systems each terminating in la pair of spaced-apart pole pieces arranged to describe a common magnetic gap and to form a socket having a recess substantially co-extensive with said gap; said subassembly comprising a supporting member, a bearing of an elastomeric material having a channel therein retained within said supporting member, an armature compri-sing a permanent magnet operatively mounted in said bearing channel for angular vibration substantially about a point intermediate the poles of said magnet, and means including a stylus connected with said armature for transmitting vibrations thereto, the portion of said supporting member containing said armature and said bearing being contoured so that it may be inserted axially into the recess of said socket at said common gap and, when so inserted, orients and retains said armature within said gap in operating position.

8. An armature-stylus subassembly according to claim 7 in which said support member has an enclosed hollow space axially behind said magnet, a damper vane mounted on said magnet in said space, and pasty substance iilling said space.

beneath said top portion; said said opening and out ofsaid support member.

9. An armaure-stylus subassembly according to-claim i7 in which said support member" has a top portion extending along and over said stylus, -andhaving an opening stylus extending through l0. An armature-stylus subassembly for a magnetic pickup cartridge comprised of 'two electromagnetic .systems each terminating in a pair of spaced-apart pole pieces arranged to describe a common magneticA gap and'to form a socket lhaving a recess substantially co-exten'sive rwith said gap, said subassembly comprising a supporting member, Ia'bearing of an elastomeric material having a channel `therein retained within said supporting'member, an armagnet operatively for angular vibration mature comprising a permanent mounted in said'bearing channelY ,substantially about ya point intermediate the poles vof said magnet, and means including a'stylus connected with said armature for transmitting vibrations thereto, the portion ofl said supporting member containing said armature and said bearing being contoured so that Ait may be Vinserted axially into 'the recess of 'said socket at said common gap, and when so inserted, orients and retains-said armature within said gap in operating position, the magnetic axis of said armature being parallel to ,the axis of the portion of said supporting member containing said armature.

l1. An armature-stylus subassembly for a magnetic pickup cartridge comprised of two electromagnet systems each terminating in a pair of spaced-apart pole pieces arranged to describe a common magnetic'gap and tolforrn a socket having a recess substantially co-extensive with said gap, said subassembly comprising a generally sheetform supporting member, a bearing of an elastomeric material `having a channclvthereinretained within said supporting member, an armature comprising a permanent magnet operatively mounted in said bearing channel for angular vibration substantially about a point intermediate Vthe poles of said magnet, and means including a stylus connected with said armature for transmitting vibrations thereto, the portion of said supporting member containing said armature and said bearing being contoured so that it may be inserted axially into the recess vof said socket at said common gap and be frictionally retained therein, and, when so inserted, orients and retains said armature within said gap in operating position.

l2. An armature-stylus subassembly for a magnetic pickup cartridge comprised of two electromagnet systems each terminating in a pair of spaced-apart pole pieces arranged to describe a common .magnetic gap and to form a socket having a recess substantially co-extensive with said gap, said subassembly comprising a supporting mein'- ber having a tubular portion, a bearing of 4an elastomeric vmaterial having a channel therein retained withinV the tubular portion of said supporting member, an armature comprising a permanent magnet operatively ,mounted in said bearing channel for angular vibration substantially about a point intermediate the poles of said magnet, 'and means including a stylus connected with said armature for transmitting vibrations thereto, the tubular portion of said supporting member being contoured so that it may be inserted axially into the recess of said socket at said com mon gap, and, when so inserted, Orients and retains said armature Within said gap in operating position.

13. A subassembly according to claim l2 wherein the tubular portion of said supporting member has a square cross-sectional configuration.

14. A phonograph pickup cartridge comprising a cartridge body and a transducer system operatively mounted therein, said transducer system comprising a pair of electromagnets each comprised of a core terminating in a pair of spaced-apart pole pieces forming a gap therebetween and a coil arranged about said core, the orientation of said pole pieces being such that they form a common gap with the gap spacing directions of the ref removable and insertable n the axial man fspec'itivepairs of pole pieces perpendicular to each other, and an armature-stylus subassembly removably positioned 1in said gap, said subassembly comprising a supporting "member'having a thin-walled sleeve at one end mounted y within said common gap, said sleeve having a snug friction fit with said pole pieces, a bearing of an elastomeric :material having an axially positioned aperture xedly :mounted within said ferrule, an armature comprising a v*permanentmagnet operatively mounted in said aperture tfor' angularvibrationwithin said common gap substant'lally about a point within said magnet, the magnetic axis of saidk armature being oriented substantially perpen- `dicular to the gap spacing directions of vsaid pairs of pole pieces, and a stylus affixed to said armature for transmittiiigyibration thereto. l p p x i5. An armaturest'ylus subassembly for a magnetic pickup cartridge comprised of two electromagnetic systems Aeach terminating in a pair of spaced-apart pole pieces arranged to describe a Vcommon magnetic gap and to form a socket having a recess substantially cto-extensive with said gap; said subassernbl'y having an elongated slim body porti-on adapted to bethrust end rst fully into said socket 4and to have a snug friction fit therein, said subassembly having four dat sides defining a square cross-section, a slim elongated permanent magnet, having poles at opposite ends, positionedwithin and along the axis of the body, andan elastomeric bearing engaging the permanent magnet substantially around its mid portion, and leaving 'its ends free for universal pivoting of the permanent magnet about a point betweenv its poles to cause its opposite poles'to approach close to opposite ilat sides of the body, fand means connected to the permanent magnet through the end of the body and including a stylus extending :laterally of the axis of the body in a plane diagonally disjposed with respect to the square cross-section.

16. Electromagnetic phonograph pickup for stereolphonic grooved records, comprising two mutually inde-` pendent magnetic circuits each having a yoke with two pole portions spaced fromeach other in a' direction intersecting the spacing directiontoffthe other two pole portions, and formingvtogether therewitha four-pole gap space, induction coils mounted on said respective yokes, a stylus assembly having an elongated stylus arm and an elongated and longitudinally magnetized permanent magnet firmly and coaxially joined with each other with the arm extending from an end of said magnet and having a stylus tip on said arm remote from said magnet, said assembly asv a whole being exchangeably seated in said gap space in coaxial relation thereto and being slidingly direction, said magnet being resiliently mounted in an elastomeric bearing member for deflection in angular relation to the gapfspace axis about a predetermined pivot point within said gap space and within said magnet, said two pole portionsof each of said yokes extending straight Ialong the entire of said magnet.

of, said sleeve having a neath said top portion, said 'magnetic circuits each having klv17. Electromagnetic phonograph pickup v for stereophonic records, comprising two mutually independent sembly having an elongated stylus arm and an elongated `and longitudinally'magnetized permanent magnet firmly joined with each other'in coaxial relation and havinga stylus tip on said arm remote from sai `i fnagr1'et,l said stylus assembly comprising a non-magnetic sleeve snrrounding said magnet, and spacer means of resilient material disposedbetween saidl magnet and said. sleeve and holding said magnet spaced from the sleeve perimeter i'n coaxial relation thereto, said magnet and arm being ani 'gularly dellectable relative to said sleeve, said sleeve being exchangeably seated in said 'gap space in coaxial relation thereto'whereby said stylus assembly is removablev and insertable into said gap space in the axial direction there top portion extending along 's aid arm and over said stylus tip, and having an opening bestylus Itip extending through said opening out of said sleeve.

18. Electromagnetic phonograph pickup for stereophonic records, comprising two mu'tually independent a yoke with two pole portions spaced from each other in a direction intersecting the spacing' direction of the other tWo pole portions and forming together therewith a four-pole gap space, induction coils mounted on said respective yokes, a stylus assembly having an elongated stylus arm and an elongated and longitudinally magnetized permanent magnet firmly joined vwith each other in coaxial relation and having a stylus tip on said arm remote from said magnet, said stylus assembly comprising a non-magnetic sleeve surrounding said magnet, and spacer means of resilient material disposed between said magnet and said sleeve and holding said magnet spaced from the sleeve perimeter in coaxial 'relaton thereto,"said magnet and Aarm being, angularly deilectable relative tosaid sleeve, said sleeve being exchangeably seated in said lgap space in coaxial relation thereto wherebyv said stylus assembly is removable and insertable into said gap space in the axial direction thereof, said sleeve having an enclosed hollow space axially behind said magnet, a damper vane mounted on said magnet in said space, and pasty substance lling said space.

References Cited in the file of this patent UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFICATE 0E CORRECTION Patent No.. 3,077,521 February l2, 1963 Erhard Ahrens et aL,

It is hereby certified that error appears in the above numbered patent requiring Correction and that the said Letters Patent should read as corrected below.

Column 7, line 56, after "entire" insert llength Signed and sealed this 3rd day of September 1963o (SEAL) Attest:v

ERNEST w. swIDEE DAVID L- LADD Attesting Officer Commissioner of Patents 

3. A PHONOGRAPH PICKUP CARTRIDGE COMPRISING A CARTRIDGE BODY AND A TRANSDUCER SYSTEM OPERATIVELY MOUNTED THEREIN, SAID TRANSDUCER SYSTEM COMPRISING A PAIR OF ELECTROMAGNETS EACH COMPRISED OF A CORE TERMINATING IN A PAIR OF SPACED-APART POLE PIECES FORMING A GAP THEREBETWEEN AND A COIL ARRANGED ABOUT SAID CORE, THE ORIENTATION OF SAID POLE PIECES BEING SUCH THAT THEY FORM A COMMON GAP WITH THE GAP SPACING DIRECTIONS OF THE RESPECTIVE PAIRS OF POLE PIECES PERPENDICULAR TO EACH OTHER, AND AN ARMATURE-STYLUS SUBASSEMBLY, SAID SUBASSEMBLY COMPRISING A GENERALLY SHEET-FORM SUPPORTING MEMBER HAVING A SLEEVE AT ONE END MOUNTED WITHIN SAID COMMON GAP, A BEARING OF AN ELASTOMERIC MATERIAL HAVING AN AXIALLY POSITIONED APERTURE FORMING A SOCKET FIXEDLY MOUNTED WITHIN SAID SLEEVE, AN ARMATURE COMPRISING A PERMANENT MAGNET OPERATIVELY MOUNTED IN SAID SOCKET FOR ANGULAR VIBRATION WITHIN SAID COMMON GAP SUBSTANTIALLY ABOUT A POINT INTERMEDIATE THE POLES OF SAID MAGNET, THE MAGNETIC AXIS OF SAID ARMATURE BEING ORIENTED SUBSTANTIALLY PERPENDICULAR TO THE SPACING DIRECTION OF SAID PAIRS OF POLE PIECES, AND A STYLUS AFFIXED TO SAID ARMATURE FOR TRANSMITTING VIBRATION THERETO, WHEREBY SAID SUBASSEMBLY IS REMOVABLE BY GRASPING SAID SUPPORTING MEMBER AND RETRACTING SAID SUBASSEMBLY AXIALLY. 